1. Field of the Invention
The present invention relates to a variable wavelength light source apparatus and an optical amplifier using the same, which are used in various optical communication systems, and in particular, to a variable wavelength light source apparatus capable of changing continuously wavelengths of a plurality of oscillation light over a wideband, and an optical amplifier constituting a pumping system using such an apparatus.
2. Description of the Related Art
For example, in a wavelength division multiplexing (WDM) optical fiber communication system, an optical amplification technique is one of key technologies, and an erbium (Er) doped optical fiber amplifier (EDFA) and the like are typically used in conventional systems. Further, as the Internet comes into widespread use in recent years, demand for networks increases explosively and an optical fiber communication system is required to have larger capacity and a longer distance. A Raman amplifier becomes practical as a main optical amplification technique for realizing such requirements. By using the Raman amplifier and the EDFA together, it becomes possible to realize a transmission characteristic of higher quality than that in the case where the EDFA is used alone, and therefore the Raman amplifier is expected to be indispensable technique in a long-distance transmission system.
For the Raman amplifier mentioned above, there are two amplification types: a distributed parameter type and a concentrated type. The distributed parameter type is the one in which pumping light is introduced into a transmission path (for example, a silica-based fiber and the like) of an optical communication system to Raman amplify distributively an optical signal being propagated in the transmission path, so that a part of transmission losses is compensated. On the other hand, the concentrated type is the one in which the pumping light is introduced concentratingly into a medium having higher non-linearity (for example, a silica-based fiber having a smaller effective cross-sectional area) to Raman amplify the optical signal efficiently. It is known that the Raman amplification described above includes characteristics having a gain peak at a frequency lower than a frequency of the pumping light by 13.2 THz in the case where the silica-based fiber is used as a medium. Therefore, the Raman amplifier has advantages capable to amplify an optical signal of arbitrary frequency according to a wavelength of pumping light. Further, by adopting a pumping system configuration in which a plurality of pumping light sources of different wavelengths are prepared and each pumping light is multiplexed to be supplied to an amplification medium, it becomes possible to realize a wider Raman amplification bandwidth, and such a Raman amplifier also becomes practical.
As pumping system components in the Raman amplifier as described above, typically, a semiconductor laser or the like is used as a pumping light source, and an interference film, a fused coupler or a Mach-Zehnder type optical filter is used as a multiplexer for multiplexing pumping light of each wavelength. For such a Raman amplifier using such pumping system components, in the case where the pumping wavelength is made to be variable over a wideband (for example, several nm or more) so as to enable the amplification of an optical signal of an arbitrary wavelength band, needless to say, multiplexing wavelength characteristics of the multiplexer is also required to be variable in accordance with a change in the pumping wavelength.
Conventional multiplexers having multiplexing wavelength characteristics variable are well known in Japanese Unexamined Patent Publication No. 56-113102, Japanese Unexamined Utility Model Publication No. 60-104804, and the like. Each of such conventional multiplexers is configured to utilize the interference film, in which, specifically, films having different transmission characteristics are deposited on the same substrate and a predetermined film is selected by making the substrate movable.
However, since each of the conventional multiplexers described above cannot change multiplexing wavelength characteristics continuously as if arbitrary wavelengths, even if the Raman amplifier having a pumping wavelength variable is constituted using the conventional pumping system components, it is practically difficult to have wavelengths of a plurality of pumping light variable continuously over a wideband.
In an optical network system of next-generation, for example, it is assumed that system operating conditions such as a signal band, the number of signals, a signal input level and a type of a transmission path may be changed dynamically. Therefore, for a Raman amplifier applied to such a system, in order to maintain a good transmission quality of each signal channel, it is required that a spectrum of pumping light (specifically, the number of peak wavelengths, a center wavelength, bandwidth, pumping light power, and the like) can be optimized accurately according to the system operating conditions that are changed dynamically.
On the other hand, with regard to a pumping wavelength control of the Raman amplifier, Japanese Unexamined Patent Publication No. 2001-235772 discloses that the pumping wavelength is made variable. However, the means for having the pumping wavelength variable in this known technique, is to change the pumping wavelength by adjusting an operation temperature of a pumping light source and the variable width of the pumping wavelength is specifically a narrow band of on the order of 0.1 nm/xc2x0 C. Therefore, it is still difficult to have the pumping wavelength variable continuously over a wideband of on the order of several nm or more as described above, and it is also still difficult to flexibly cope with the system operating conditions that are changed dynamically.
In view of the above problems, it is an object of the present invention to realize a variable wavelength light source apparatus capable of varying continuously a plurality of oscillation wavelengths over a wideband. Further, it is a further object of the present invention to provide an optical amplifier that can support a wide system operation range and also can cope with a change in system operating conditions smoothly by constituting a pumping system using the variable wavelength light source apparatus as described above.
In order to achieve the above objects, a variable wavelength light source apparatus according to the present invention, for multiplexing a plurality of variable wavelength light to output multiplexed light, comprises: a wavelength selection device in which a propagation direction of emitted light is changed according to a wavelength of incident light; a plurality of light source sections, each including a gain medium that amplifies light and a reflection component that reflects the light incident on one end and emitted from the other end of the gain medium, to return the light to the other end, and emitting the light that has reciprocated in the gain medium and has been amplified, to a predetermined position of the wavelength selection device at angles different from each other; an optical resonance reflection section including a light incident surface on which the emitted light from the wavelength selection device enters, that reflects a part of the light incident vertically on the light incident surface to form an optical resonator configuration between the optical resonance reflection section and each of the reflection components of the light source sections, to generate oscillation light; an optical coupler section coupling the oscillation light transmitted through the optical resonance reflection section in an output light path; and a wavelength selection device drive section changing an arrangement angle of the wavelength selection device with respect to the optical resonance reflection section with the predetermined position as a center.
In the variable wavelength light source apparatus of the above constitution, since the light emitted from each light source section enters the predetermined position of the wavelength selection device at the different angles and is emitted toward the light incident surface of the optical resonance reflection section, and only the light incident vertically on the light incident surface of the optical resonance reflection section is reflected and resonates between the optical resonance reflection section and each of the reflection components of the light source sections to oscillate. A wavelength of each light that oscillates by each optical resonator configuration between the optical resonance reflection section and each of the reflection components of the light source sections is made variable by changing the arrangement angle of the wavelength selection device with respect to the optical resonance reflection section by the wavelength selection device drive section. Thus, the light that has been transmitted through the optical resonance reflection section and introduced through the optical coupler section to the output light path becomes light in which a plurality of oscillation light, wavelengths of which can be changed continuously over a wideband, is multiplexed.
Further, the variable wavelength light source apparatus described above may comprise a light source drive section changing an emission angle of the light directed to the predetermined position of the wavelength selection device, for at least one or more of the plurality of the light source sections. Thus, an interval between the oscillation wavelengths corresponding to each light source section can be varied.
Still further, the variable wavelength light source apparatus described above may comprise a monitoring section detecting a wavelength of the oscillation light coupled in the output light path, and a control section controlling the wavelength selection device drive section or the light source drive section according to the wavelength detected by the monitoring section. In such a constitution, since the arrangement angle of the wavelength selection device and the light emission angle of each of the light source sections are feedback controlled according to the oscillation wavelength of the multiplexed light that is actually output from the output light path, and fluctuation of the oscillation wavelengths due to environmental variation and the like is corrected, light of a desired oscillation wavelength can be output stably.
The variable wavelength light source apparatus according to the present invention as described above is suitable, for example, as a pumping light source for various optical amplifiers such as a Raman amplifier. According to such optical amplifiers, since wavelengths of a plurality of pumping light can be changed continuously over a wideband, amplification of signal light operated in a wide wavelength band can be realized by a single pumping light source.